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of which is too small either to communicate its heat to it, or to create an obstacle to the free circulation of the air, when the thermometer is fixed at a due distance from it. I have always followed the same plan. A staff of about six feet in length answers the purpose; it has an iron point to fix it in the ground. At the other end are two holes to receive two small arms of iron or brass, about five inches long; one terminated by a hook, the other by a ring, the end of each which is to fit into the hole is formed into a screw. The arm with the hook is fixed in the upper hole, and serves to suspend the thermometer; that with the ring in the lower, and retains it in a position parallel with that of the staff. The staff serves for myself or my guide, and the arms are carried in the bag which holds the thermometers. I do not believe it possible to attain the requisite precision in the indications with less apparatus.

The observation of the thermometers is the most delicate and the most difficult part of the operations; and most of the faults which we commit in the measurement of heights may be traced to a false valuation of the temperature of the air, or of that of mercury. I have mentioned this before, but there is no harm in repeating it, and we cannot be too careful in pointing out the sources of error, especially when they are of such a nature as easily to disguise themselves to inattentive eyes. The inexperienced observer, when he meets with unsatisfactory results, will be less tempted to lay the blame on his instruments, or on his formula; and this may often spare men of speculative minds the trouble of imagining new theories to correct in the formula, irregularities, which exist only in the observations.

All philosophers who have been engaged in barometrical measurements must doubtless have made the same remark which I have. They cannot have carried the thermometer to summits of mountains, exposed to all winds, without often experiencing the same embarrassment which I have. The thermometer has varied, with them as well as with me, every instant in proportion to the degree of wind, of calm, the presence of the sun, or the interposition of clouds. These variations they have not neglected, because they could not misinterpret them. Like me they have often been in uncertainty as to the real and intrinsic temperature of the air, and that of their instruments; and have not assuredly confined themselves to noting down at all hazard merely the temperature which any accident may cause to prevail at the exact moment of observation. But if they are generally silent on a point which cannot escape any experienced person, it is that they suppose the logic of the observations familiar to all those who employ meteorological instruments with any discernment. I conceive I ought not to imitate them in addressing myself to beginners. Such readers require advice and examples. I will give one or two, and I do not select the most rare

cases.

Aug. 30, 1805.-On the summit of the Pic de Midi, between 10, a.m. and 1, p.m. the thermometer varied from 14° to 19°. This was owing to an irregular wind. In moments of calm, it stood at 16° or 17°, this being partly caused by heat of the surface it fell when the wind brought against it masses of colder air; and rose to 18° or 19° when the current continued. In this complication of influences, the only way was to take a mean between the extremes; for it would have been equally absurd to keep to either 14°, 16°, or 19°, for the sole reason that the thermometer had accidentally indicated one of these degrees at the precise moment when we noted the height of the barometer.

Sept. 11.-On the Pic de Bergons, there was a brisk wind subsiding at intervals. The temperature of calm was above 14°, but the ground reflected much heat. It is seldom that this is not the case with the observed temperature of a calm air. This temperature, however, increased as the wind rose: it was a different atmosphere which the wind drove before it, heated by passing over the neighbouring plains. It rose to 15.5°, but when the wind continued constant, sunk to about 13°, and there became stationary, which was certainly the true temperature of the air under the prevailing modification, the higher temperatures were transient and accidental.

Aug. 10, 1802. On the summit of Mont Perdu, a thermometer, placed on the snow, sunk to 2.5°, owing to the rapid evaporation. Another suspended at the height of about 4 feet partook of the same influence, and never rose above 4° or 5o. Another at the same time suspended above a rock free from snow stood at 12.5°, and one placed on the rock at 22.9°. The continuance of a wind (which at first brought hot air from the plains, gradually reduced them all to about 7.5°, nearly the mean; this was the true temperature of the air.

It is superfluous to mention a multitude of other cases differing but little from these; such as a passing shower which causes a variation in the thermometer the moment it touches it: a local fog, which occasions, in the particular atmosphere of the instruments, a cooling, in which the rest of the stratum of air does not partake the influence of the sun which raises the thermometers; the intervention of clouds which makes them sink; all the variations which originate in reverberations or absorptions of heat, or in currents of air which are accidental and of limited influence; every thing which conspires to alter the general temperature of unfavourable situations, such as deep valleys, and even eminences above which greater heights immediately rise. I have said enough to awaken and direct the attention of those who wish, and know how, to be exact.

Such are the considerations which must guide us in reference to the thermometer for the air. We have just seen that the thermometer for the barometer is the object of considerations very different; for it, the temperature of the air is only a matter

of secondary importance. It is always well placed wherever the barometer is placed. Its variations are of no consequence, provided the temperature of the barometer undergoes the same changes; but this accordance is the thing of which we must be careful to assure ourselves, and this is the difficult point. It is very seldom that the two instruments, joined together as they usually are, will preserve this accordance when the temperature undergoes great and frequent variations. We may diminish the sensibility of the thermometer; but in thus preventing its indications from outstripping the rapidity of the changes which the temperature of the barometer undergoes, we must take care not to approach the point where, on the other hand, it may experience a retardation; and in all cases there is a wide field open to doubt and conjecture.

We should commence our operations by disposing the instruments in a convenient manner, and should then allow them time to lose the heat which they have contracted in carrying, and to acquire, each in its proper manner, the local temperature. This time is considerable for barometers cased in wood, which become unequally heated in the hands, or on the backs, of those who carry them. The heat thus acquired never distributes itself uniformly, and is dissipated with extreme slowness. Often an hour does not suffice to bring the barometer to an agreement with the thermometer, and with itself. More than a quarter of an hour is not necessary for barometers mounted in copper, and this is one reason for giving them the preference.

The time thus occupied, however, is not lost: we can make trial of the position: it may not always be suitable for the barometer: if it be too much exposed to wind, we can seek for a shelter. At the summit of a mountain, a strong wind has other inconveniences besides that of agitating the instrument: it rises on the acclivity against which it directs itself, and forms an ascending current, which bears up the column of air, and depresses that of the mercury. In such a situation the thermometer should be left; but the barometer should be removed from this part of the acclivity, and if the summit has not sufficient extent, we should leave it, and seek for a calm under the shelter of the opposite side, only being careful to allow for the quantity of our descent: then we shall await the moment of observation in considering the changes of the thermometers. We shall remark attentively how the one is affected in the free air, and the other in the mounting in which it is inclosed; whether they undergo considerable and frequent variations, or whether they reduce themselves by slow degrees to a stationary point. In this last case, we shall have no occasion to doubt the success of the operation: in the former, we shall examine the causes of the variations: we shall take an exact account of the nature and influence of every accidental circumstance. We shall repeat this examination after the observation in order to

state every thing which we have been able to ascertain with the greatest certainty during the short period of time of which it occupies the middle. The rule for the measurement of heights supposes that we know with great precision the temperature of the air, and that of the instrument: both these then we must discover, and when they disguise themselves, must divest them of their disguises. There are times when this is altogether impossible; but it is at least something to perceive this, and to know that we have to doubt an observation, well performed in itself, but of which we cannot be sure that we have entirely satisfied some of the fundamental conditions.

Under some circumstances, and those very common, the local temperature is so predominant, that in spite of all our care, it will enter all our estimates: it is in vain to guard against it. Measurements made in a hot season, and under a burning sun, always tend to err in excess; especially if the station be of such a nature as to multiply the reverberations of heat. On the other hand, they will err in defect in foggy or rainy weather, especially if the place is so circumstanced as to concentrate the cold. The error proceeds from the thermometers. To point out its origin is to warn the observer to avoid, if he can, the circumstances which produce it; and if he has not the choice, to allow for their influence in the opinion he forms of his measurements. (To be continued.)

ARTICLE II.

A List of Substances arranged according to their Thermoelectric Relations, with a Description of Instruments for exhibiting Rotation by Thermoelectricity. By the Rev. J. Cumming, MA. FRS. and Professor of Chemistry in the University of Cambridge.

(To the Editor of the Annals of Philosophy.)

Cambridge, July 23, 1823.

DEAR SIR, THE following tables will, I hope, be interesting to those who have read my communication to you in April last. The first contains the thermoelectric relations of different substances, with copper wires; the second, their relations to each other taken two and two together, each substance being positive to all below, and negative to all above. The voltaic series, and the order of conductors of electricity and heat, are added, merely to show that the thermoelectric series has no accordance with either of them.

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Electromagnetic Relations of different Substances with Copper Wires; the Magnitude of the Substance examined being greater than that of the Copper Wire, excepting those marked*, none of which exceeded half a Grain in Weight.

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+ The compound of bismuth and zinc, it is well known, is not an alloy, yet it acted negatively whether the heated part appeared to be zinc or bismuth. The compound ore of iridium and osmium was from Dr. Wollaston: the alloys of nickel from Dr. Clarke, formed by the gas blowpipe.

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